(1) Field of the Invention
The present invention relates to thermoplastic polymer foams prepared from starch-polyester graft copolymers. The present invention particularly relates to foams prepared with a chemically modified starch-polyester graft copolymer or a chemically modified starch-nano clay polyester graft copolymer and a thermoplastic polymer other than the copolymer. A preferred process of making these thermoplastic polymer foams using extrusion is disclosed. The foams have improved processability, surface properties, and an extended range of mechanical properties.
(2) Description of Related Art
Plastic packaging is subjected to pressure from existing and proposed environmental and disposal regulations, and market based sustainability initiatives. It presents a major disposal problem for companies and municipalities as it is lightweight and bulky and so does not lend itself to a viable economic and environmentally responsible recycling operation due to expensive handling and transportation costs. It is not biodegradable, which makes disposal in soil or composting operations untenable. Further, issues such as sustainability, industrial ecology, biodegradability, and recyclability are becoming major considerations in a company's product packaging design, especially with single use disposable packaging.
Natural biopolymers provide biodegradable, sustainable solutions for the manufacture of short-life, single use disposable packaging, consumer goods, and marine plastics. Starch, an anhydroglucose polymer, offers a structural platform to manufacture sustainable, biodegradable packaging. Examples of patents that disclose the manufacture of starch and blends of starch and other polymers include U.S. Pat. No. 4,673,438 to Wittwer et al.; U.S. Pat. No. 4,095,054 to Lay et al.; U.S. Pat. No. 5,256,711 to Tokiwa et al.; U.S. Pat. No. 5,275,774 to Bahr et al.; U.S. Pat. No. 5,382,611 to Stepto et al. and U.S. Pat. No. 5,405,564 to Stepto et al. Lately, there have been business reports of the use of thermoplastic starch (TPS) as a component in multi phase blends (W. Wiedmann, and E. Strobel, Starch, 43, 138 (1991); R. L. Shorgen, G. F. Fanta, and W. M. Doan, Starch, 45, 276 (1993); P. Forssell, J. Mikkila, and T. Sourtti, J.M.S. Pure Appl. Chem., A33, 703 (1996); R. Narayan, Polymers from Agricultural Co products, ACS Symp Ser. (1994); and J. J. G. Van Soest, K. Benes, and D. de. Wit, Polymer, 37, 3543 (1996). Still others have manufactured thermoplastic starch blends in which native starch is initially blended with a small quantity of water and a less volatile plasticizer such as glycerin in order to form starch melts that are subjected to a degassing procedure prior to cooling and solidification in order to remove substantially all of the water therefrom. Examples of such patents include U.S. Pat. Nos. 5,412,005, 5,280,055, 5,288,765, 5,262,458, 5,462,980 and 5,512,378 to Bastioli et al.
Starch granules exhibit hydrophilic properties and strong inter-molecular association via hydrogen bonding due to the hydroxyl groups on the granule surface. The hydrophilicity and thermal sensitivity render the starch polymer unsuitable for thermoplastic applications. In this respect, some authors have emphasized on finding the optimal polymer or mixture of polymers and other admixtures in order to thereby “optimize” the properties of the starch. One drawback is that most of the polymers and other admixtures are themselves significantly more expensive than starch, which tend to increase the cost of such polymer blends compared to starch melts. Another drawback is that such additives will only be able to marginally alter the mechanical properties of the starch/polymer blends when viewed from a materials science perspective.
In order to improve on these drawbacks, graft copolymerization of vinyl monomer on the starch backbone was used to modify starch. Fanta and Bagley have reviewed the synthesis and discussed some applications of starch graft copolymers (G. F. Fanta and E. B. Bagley, Encyclopedia of Polymer Science, John Wiley & Sons: New York (1970); and G. F. Fanta, Block and Graft Copolymers—Vol I, John Wiley & Sons: New York (1973). Otey et al. (F. H. Otey, R. P. Westhoff and W. M. Doane, Industrial Engineering Chemistry Products Research Development, 19, 592 (1980); F. H. Otey and R. P. Westhoff, Industrial Engineering Chemistry Products Research Development, 23, 284 (1984); and F. H. Otey, R. P. Westhoff and W. M. Doane, Industrial Engineering Chemistry Products Research Development, 26, 1659 (1987)) blended starch with poly(ethylene-co-acrylic acid) (EAA). In these papers, the authors suggested the formation of hydrogen bonds between the carboxylic group in EAA and the hydroxyl group in starch. Increasing the level of starch decreased the percentage elongation of the film and increased the diffusion rate of water. Similar complexes like EAA can also be formed with the hydroxyl groups of the polyethylene-vinyl alcohol (EVOH) copolymer. U.S. Pat. No. 5,462,983 to Bloembergen at al. reports on blends and alloys containing lignocelluloses like starch, cellulose acetate and the like. U.S. Pat. No. 5,314,934 to Tomka et al. provides a process to produce a polyolefin-starch polymer blend. Ethylene/acrylate/maleic anhydride terpolymer was used as a compatibilizer. These blends were reported to be blown into film with properties comparable to low density polyethylene (LDPE). U.S. Pat. No. 5,234,977 to Bastioli et al. discloses a material used for the production of biodegradable articles in film, sheet or fiber form, which can be produced by extrusion from a molten mass that includes a synthetic thermoplastic polymer and a destructured starch to which a boron containing compound such as boric acid has been added. U.S. Pat. No. 6,277,899 to Bastioli et al. discloses a polymeric composition comprising filler melt-dispersed in a matrix comprising, a destructurized starch component, a synthetic thermoplastic polymeric component and a fluidizing agent. U.S. Pat. No. 5,412,005 to Bastioli et al. discloses biodegradable polymeric compositions containing a starch based component and a polymeric component, preferably polymers of ethylene-vinyl alcohol or polyvinylalcohol.
U.S. Pat. Nos. 6,235,816 and 6,472,497 describe starch polyester blends.
Other references are:    (1) Ramani Narayan, Steven Bloembergen and Amit Lathia, A Method of Preparing Biodegradable Modified-Starch Moldable Products and Films, U.S. Pat. No. 5,869,647, Feb. 9, 1999, July 1993;    (2) Narayan, R., Biodegradable Multi-Component Polymeric Materials Based on Unmodified Starch-Like Polysaccharides, U.S. Pat. No. 5,500,465, Oct. 31, 1995;    (3) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,540,929, Jul. 30, 1996;    (4) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,578,691, Nov. 26, 1996;    (5) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,616,671, Apr. 1, 1997;
Starch-based foams have been developed before using biodegradable polymers as additives/processing aids in order to impart better physico-mechanical properties such as lower density, better resilience, higher hydrophobicity.
U.S. Pat. No. 4,863,655 (1989), U.S. Pat. No. 5,035,930 (1991) and U.S. Pat. No. 5,043,196 (1991) all issued to N. Lacourse et al, showed the extrusion of starch having high amylose content of at least 45% by weight into expanded packaging products. A difficulty with the product described by U.S. Pat. No. 4,863,655, is that the expanded amylase starch material is not, by itself, suitable for preparing containers where moisture resistance is a necessary property (e.g. various take-out food containers).
U.S. Pat. No. 5,095,054 issued to G. Lay et al (1992) discloses thermoplastic polymer compositions comprising destructurized starch with a variety of thermoplastic polymer materials. Processing aids such as polyolefins, polyesters, polyethers, polycarbonates, polyacetals, vinyl polymers, etc. The patent further discloses processes for forming such compositions into shaped articles. Although these blends and articles made therefrom are said to show an improved dimensional stability in humid air when compared with non-blended destructurized starch, nevertheless the articles are said to retain a high degree of disintegration in contact with liquid water. Although this disintegration may assist in disposal of such articles in landfills and the like, the high degree of disintegration in contact with liquid water is disadvantageous for many applications where substantial moisture is present.
U.S. Pat. Nos. 5,185,382 and 5,208,267 issued to P. Neumann et al (1993) discloses a foamed starch product, using wheat/corn starches with polyalkylene glycols or derivatives thereof.
U.S. Pat. No. 5,272,181 issued to Boehmer et al (1993) a starch-graft polymer is combined with a starch to form a composition which may be expanded into foam products ranging from rigid to resilient. The starch-graft copolymer was obtained which had been manufactured in accordance with U.S. Pat. No. 4,026,849. This starch-graft copolymer was prepared by grafting a monomer methyl acrylate onto an unmodified basic corn starch (20% amylose) under relatively severe conditions. These materials are said to readily disintegrate under wet conditions. Although such disintegration may assist in integrating the materials into soil, the disclosed materials are not entirely biodegradable due to the presence of the synthetic resins (such as polyacrylate).
U.S. Pat. No. 5,288,765 (1994); U.S. Pat. No. 5,360,830 (1994); U.S. Pat. No. 5,736,586 (1998); and U.S. Pat. No. 5,801,207 (1998) granted to Bastioli et al disclose starch foams made by using 10-30% of polymers such as PVA, poly(caprolactone), cellulose acetate, poly(ethylene vinyl alcohol), and poly(ethylene-co-acrylic acid).
U.S. Pat. No. 5,554,660 issued to P. Altieri et al (1996) discloses an expanded foamed starch product with improved water/humidity resistance comprising the combination of starch/modified starch with starch esters.
U.S. Pat. No. 5,496,895 issued to Chinnaswamy et al (1996) discloses biodegradable polymers and methods of making them from non-biodegradable polymers such as petroleum-based plastics combined with other biodegradable polymers, such as for example, carbohydrates, proteins, lipids or the like. These formulations were used for manufacturing loose-fill.
U.S. Pat. No. 5,665,786 (1997) and U.S. Pat. No. 5,854,345 (1998) issued to Xu et al disclose expanded articles having a compressible and resilient body including starch or a modified starch and a water insoluble, synthetic polymer, preferably a hydroxyl-functional polymer.
U.S. Pat. No. 6,184,261 granted to Biby et al discloses a starch foam using a biodegradable polymer that is selected from the group consisting of poly(tetramethylene adipate-co-terephthalate) and a resin comprised of 10-50% by weight ethylene acrylic acid copolymer.
Objects
It is an object of the present invention to provide foams with good compatibility between the starch and the polyester, providing better physical properties such as density and surface finish, and mechanical properties such as compressive strength and resilience. Further, it is an object to provide processable products, which are also biodegradable. Further still, it is an object to provide new starch-based foam materials, which utilize agricultural resources and return those resources to nature in an environmentally sound manner. It is further an object of the present invention to provide foams comprising novel grafted starch polyester compositions which have novel properties. It is further an object of this invention to provide economical and reproducible compositions. These and other objects will become increasingly apparent from the following description.